Chapter 10 Memory Subsystem.pdf

Class 1 and class 2 request for transaction
(requests previously received on group 1 and group 2).
An arbitration decision occurs.
Class 1 has less priority than class 2. Return to
class 1 - group 1 after the service of class 2 - group 2.
Group 0 Group 1 Group 2 Group 4 Group 7
4 x 64-bit burst
transaction
ongoing
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1. A thread requests a burst transaction. According to the thread ID, the last request was serviced on
Class 2 - Group 2.
2. The thread cannot provide the subsequent request of the burst. The module waits for one idle cycle
without moving the arbitration grant, to receive the subsequent request (from the same thread).
3. On the second idle cycle, if the subsequent request has not been received, the arbitration grant is
moved so that a request from another thread is serviced.
Figure 10-70. Idle Cycle Mechanism Within A Burst
.......
Move the arb. Grant
Wait 1 idle cycle
Request for transaction on group 4
occurs during transaction on group 2.
Group 6
Class 1 Class 2
Class 0
.......
A burst transaction is serviced on group 2: the burst is not complete.
Wait 1 idle cycle to get the subsequent request of this burst.
On the second idle cycle, if nothing has been received:
move the arbitration grant to class 1 - group 1 or class 2 - group 4.
Two bursts must be serviced at the burst boundary. One idle cycle after servicing these two bursts, the
arbitration grant is moved.
10.2.6.2.3.2 Extended-Grant Mechanism
The extended-grant mechanism gives additional granularity to the arbitration. An extended grant defines
the number of consecutive transactions (from 1 to 3) a group is granted.
Example:
Requests were already available on Class 1-Group 0 and Class 2-Group 3.
An arbitration decision occurs: grant access to Class 1-Group 0.
A request is now being serviced on Class 1-Group 0 with SMS_CLASS_ARBITER1[9:8]
EXTENDEDGRANT = 0x3 (three consecutive transactions are granted to Group 0). The mode of
operation is:
• Process the request for three cycles.
• If another request comes from another class (even Class 0) during the processing, it is ignored; the
arbiter does not treat incoming requests.
• After three consecutive transactions, return to the arbitration decision:
– Were there any incoming requests during the last transaction?
– What is the next request to be serviced?
Figure 10-71 shows this mechanism on Class 1-Group 0:
SPRUGN4L–May 2010–Revised June 2011 Memory Subsystem
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